(1) Field of the Invention
The present invention generally relates to an interconnect system. More specifically, the present invention relates to an interconnect system including a low dielectric constant material.
(2) Description of the Related Art
Modern integrated circuits are generally made up of a silicon substrate containing millions of active and passive devices including transistors, capacitors, and resistors. Such devices are initially isolated from one another, but are later interconnected together to form functional circuits. The quality of the interconnection of these devices drastically affects the performance and reliability of the fabricated integrated circuit. An interconnect system typically includes metal lines, spaced apart from each other, with dielectric layers therebetween that isolate the metal lines from one another. The metal lines typically interconnect the various active and passive devices residing in a silicon wafer to which the interconnect system is coupled.
Inherent in the structure of the interconnect system is a capacitance associated with the metal lines and the distance therebetween, i.e., inter-metal-line capacitance (hereinafter "capacitance"). Decreasing this capacitance is desirable as several advantages can be achieved therefrom, such as reduced RC delay, reduced power dissipation, and reduced cross-talk between the metal lines. As the capacitance is inversely proportional to the distance between the metal lines, one way to reduce the capacitance formed between the lines of an interconnect system would be to increase the space between these lines. However, this option is not desirable because of limitations imposed by packing density.
However, as the capacitance of an interconnect system is also directly proportional with the dielectric constant of a dielectric disposed between these lines, a better way to reduce the capacitance of the interconnect system is to utilize a dielectric material with a low dielectric constant. Typically, interconnect systems use a dielectric material such as silicon dioxide (SiO.sub.2). The dielectric constant of SiO.sub.2 is approximately 4. Other dielectric materials that are used in interconnect systems include silicon oxifluoride (SiOF), with a dielectric constant of 3.5 and below.
Incorporating dielectric materials with low dielectric constants in interconnect systems, however, is problematic. Low dielectric materials, such as organic polymers, may not adequately mechanically support the structure of the interconnect system as these materials are mechanically weak. Moreover, low dielectric constant materials such as organic polymers are vulnerable to certain processes involved in the fabrication of the interconnect system. For example, polymers have to withstand high temperatures and mechanical stresses at which the interconnect system is subjected. Also, the polymers used in an interconnect system interact with photoresist solvents and developers used in the process of fabrication of the interconnect system. Moreover, polymers are vulnerable to exposure to photoresist solvents and developers as the polymers may dissolve in the solvents or developers and swell. Furthermore, polymers are vulnerable to oxygen plasma used in ashing. Additionally, polymers may be attacked by chlorine plasma used in etching the metal lines of the interconnect system.
Also, an interconnect system typically utilizes via plugs made of tungsten (W), by way of non-limiting example, that interconnect the various layers of metal lines within the interconnect system. Once the via plugs are framed in an interconnect system, the via plugs are subjected to a process of chemical mechanical polishing (CMP), wherein a CMP slurry solution is used to polish and thereby planarize the outer parts of the via plugs. The CMP slurry solution, however, can deteriorate or contaminate the polymer. Furthermore, polymers have a relatively high water diffusion constant such that any water mixed or added to the substances used in the process of fabrication may infiltrate into the polymers.
What is needed is an interconnect system that can accommodate the above-mentioned shortcomings of polymers or materials similar thereto.